1. Field of the Invention
The present invention relates to an optical recording medium having a writable area in which grooves are formed along recording tracks and a read-only area in which pits are formed along the recording tracks, and a stamper destined for use to manufacture the optical recording medium.
2. Description of the Related Art
The optical recording media include an optical disc, magneto-optical disc, phase-change type optical disc, etc. In some of such optical recording media, there are provided a writable area having formed therein along recording tracks grooves at which information signal can be written, and a read-only area in which pits corresponding to information signal are pre-formed along the recording tracks. A typical one of the optical recording media that has both the writable and read-only areas is currently available under the name of xe2x80x9cMini Disc (MD)xe2x80x9d.
The MD is a magneto-optical disc of a recording format shown in FIG. 1. As will be seen from FIG. 1, the MD has a writable area 101 having formed therein single-spirally along recording tracks grooves g at which information signal can be written, and a read-only area 102 in which pits p corresponding to information signal are pre-formed single-spirally along the recording tracks.
In the writable area 101, there can be written information signal at the grooves g by magneto-optical recording. On the other hand, in the read-only area 102, there are pre-recorded information such as TOC (Table Of Contents), etc. by pits p. Note that it is prescribed for the MD that both track pitch T1 in the writable area 101 and track pitch T2 in the read-only area 102 should be 1.6 xcexcm.
As a next-generation magneto-optical disc, xe2x80x9cMD Data2xe2x80x9d has been proposed which is about 4 times higher in recording density than the conventional MDs and has a storage capacity of about 650 MB.
The recording format in the MD Data2 is as shown in FIG. 2. The MD Data2 has also a writable area 111 having formed therein along recording tracks two types of grooves gw and gs at which information signal can be written, and a read-only area 112 in which pits p corresponding to information signal are pre-formed along the recording tracks, as shown in FIG. 2.
In the MD Data2, the writable area 111 has formed therein double-spirally two types of grooves gw and gs. One (gw) of these grooves gw and gs is a groove wobbled correspondingly to address information while the other (gs) is a straight groove, not wobbled. Information signal is recorded on a land between the wobbling and straight grooves gw and gs.
In the writable area 111 of the MD Data2, the interval between the neighboring wobbling groove gw and straight groove gs is taken as a track pitch. Track pitch T3 in the writable area is 0.95 xcexcm. In the MD Data2, the interval between the neighboring straight grooves gs is called a track period. Track period T4 is two times larger than the track pitch T3 and it is thus 1.90 xcexcm.
On the other hand, in the read-only area 112 of the MD Data2, the pits p are formed single-spirally along the recording tracks as in the MD. The pits p record TOC information or the like. In the MD Data2, however, track pitch T5 in the read-only area 112 is 0.95 xcexcm similarly to the track pitch T3 in the writable area 111.
As mentioned above, in the MD Data2, the track pitch is defined considerably smaller than that in the MD in order to improve the recording density. However, the reduced track pitch will lead to a lower level of a written or read signal.
For example, a push-pull signal is used in the tracking servo control to allow a light spot to track a recording track during data write or read. When the track pitch is reduced, a resulted push-pull signal will have a lower level. Especially, a push-pull signal obtainable from the read-only area in which only pits are formed has originally a low level. When the track pitch is reduced as in the MD Data2, it is essential to assure a sufficient level of a push-pull signal from the read-only area as well.
Also, in case the track pitch is made smaller, the modulation degree and readout characteristics (jitter, etc.) of the pits will be worse when signals are read from the pits in the read-only area. Similarly, in case the track pitch is made smaller, the writing and readout characteristics (jitter, etc.) will be worse when signals are written to or read from the grooves in the writable area.
Therefore, for a higher recording density as in the MD Data2, not only the track pitch should be reduced but also it is necessary to assure sufficient modulation degree and readout characteristics of the pits formed in the read-only area when signals are read from the pits, and sufficient writing and readout characteristics when signals are written to or read from the writable area.
When address information is added to the wobbling grooves as in the MD Data2, it is desired that the address information can successfully be read from the wobbling grooves. That is, it is necessary to assure a sufficient C/N with which the wobbling of the wobbling grooves is detected.
As in the foregoing, for a higher recording density of an optical recording medium, not only the track pitch should be reduced but also sufficient levels of various signals required for data write and read should be assured for various characteristics.
It is therefore an object of the present invention to overcome the above-mentioned drawbacks of the prior art by providing an optical recording medium capable of assuring sufficient levels of signals required for data write and read, even with the track pitch reduced for a higher recording density as in the MD Data2.
It is another object of the present invention to provide a stamper destined for use in manufacturing such an optical recording medium.
The above object can be attained by providing an optical recording medium having a writable area in which grooves are formed along recording tracks and a read-only area in which pits are formed along the recording tracks, wherein:
on the assumption that the refractive index of a medium provided extending from a light-incident surface to a surface in which the grooves and pits are formed is n and the wavelength of a light used for data write and read is xcex, the grooves and pits has a width within a range of xcex/7.5 n to xcex/6.0 n;
on the assumption that the width of each of the grooves formed in the writable area is Wg and the pitch between the recording tracks in the writable area is Tp1, Wg/Tp1 is within a range of 18.4 to 31.1%; and
on the assumption that the width of each of the pitches formed in the read-only area is Wp and the pitch between the recording tracks in the read-only area is Tp2, Wp/Tp2 is within a range of 35.7 to 44.3%.
By defining the depth and width of the grooves and pits as in the above, it is possible to assure sufficient levels of signals required for data write and read, even with the track pitch reduced for a higher recording density.
The above optical recording medium may be adapted such that the writable area has two grooves formed double-spirally therein, and at least one of these grooves is at least partially wobbled. In this case, assuming that the amplitude of the wobbling groove is A, A/Tp1 should preferably be within a range of xc2x11.68 to xc2x12.53%. By thus defining the amplitude of the wobbling grooves, it is possible to assure a sufficient C/N with which the wobbling of the wobbling grooves is detected, even with the track pitch being reduced.
Also the above object can be attained by providing a stamper for use to manufacture the above optical recording medium and having formed thereon pit and land patterns corresponding to at least the grooves to be formed in the writable area of the optical recording medium and the pits to be formed in the read-only area of the optical recording medium.
These objects and other objects, features and advantages of the present intention will become more apparent from the following detailed description of the preferred embodiments of the present invention when taken in conjunction with the accompanying drawings.